Hydraulic tensioners have been widely used to maintain proper tension in, and prevent vibration of, a transmission medium such as a timing chain, which transmits rotation between a crankshaft and one or more camshafts in a vehicle engine to prevent vibration generated in the travel of the medium and maintain proper tension.
A typical conventional hydraulic tensioner, shown in FIG. 5, is described and depicted in Japanese Laid-open Patent Publication No. Hei-11-336855. As shown in FIG. 5, in the conventional hydraulic tensioner 500, a plunger 520 fits slidably in a plunger accommodating hole 511 formed in a housing 510, hollow interior 521, open at one end, is formed in the plunger 520, and a helical plunger-biasing spring 530, which biases the plunger 520 in the protruding direction, is disposed between a closed end of the plunger-accommodating hole in the housing and the closed end of the hollow interior of the plunger. A high pressure chamber R is formed by the plunger-accommodating hole 511 and the hollow interior 521 of the plunger 520. This high pressure chamber R is filled with oil, supplied under pressure from the engine's lubrication system, through a check valve 540.
The check valve 540 comprises a ball seat 541, a check ball 542 opposite to the ball seat, a cylindrical coil spring 543, which biases the ball toward the seat 541, and a retainer 544, which supports the spring 543, and is held against the bottom portion of the plunger-accommodating hole 511. The check valve allows oil to flow into the high pressure chamber R but blocks flow of oil in the opposite direction.
In the conventional hydraulic tensioner the retainer of the check valve extends into the helical plunger-biasing spring 530, so that the inner diameter of the plunger-biasing spring 530 is limited by the size of the outer diameter of the retainer 544. Therefore, the conventional retainer causes design problems, and prevents decreasing the diameter and reducing the weight of the plunger-biasing spring 530. The conventional retainer also prevents a decrease in the diameter and weight of the plunger 520, since the plunger receives the biasing spring 530 in its hollow interior 521. The presence of the retainer inside the plunger-biasing spring also gives rise to contact resistance due to buckling of the plunger-biasing spring inside the hollow interior 521 of the plunger 520, impairing smooth retracting and extending movement of the plunger 520. Furthermore, the available space in the high pressure chamber R is reduced because most of the check valve extends into the high pressure chamber R. Thus, the amount of oil which can be stored in the high pressure chamber R is limited by the presence of the check valve, and the limitation on the amount of oil in the chamber R adversely affects the tensioner's response characteristics.
Since most of the check valve structure extends into the high pressure chamber R, when there is a possibility that, in the assembly process, the retainer or the ball seat may not be fitted properly into the housing at bottom of the plunger-accommodating hole 511. Unless the retainer and ball seat fit properly in the housing, leakage of oil can occur, which results in troublesome problems.
The principal objects of the invention, therefore are to solve the above-mentioned problems experienced in case of prior art hydraulic tensioners, and to provide a hydraulic tensioner which can be more compact and lighter in weight, capable of smooth retracting and extending movement of the plunger, and exhibiting superior response characteristics.